Fluorescent lamps and light emitting diodes (LEDs) are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors and general lighting applications. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ionize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on.
In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCDs or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness. Even brightness across the panel and clean operation of inverters with low switching stresses, low EMI, and low switching losses is desirable. While CCFL backlighting is common, other fluorescent lamps such as external electrode fluorescent lamps (EEFLs) or flat fluorescent lamps (FFLs) may be utilized in place of CCFLs, with somewhat similar requirements. With the increasing size of LCDs and the high screen brightness requirements for better display quality, the power consumption of the backlight system becomes a major factor in the total system power consumption of an LCD based monitor or television.
In many prior art systems, the incoming power line voltage is first rectified, and a power factor corrector (PFC) is typically provided. The rectified voltage is then converted to a low voltage, typically on the order of 24 volts, and the low voltage is fed to a backlight controller. The backlight controller controls a switching network connected to the primary side of a transformer, and the fluorescent lamps are connected to the secondary side of the transformer. The backlight controller is operative to produce the necessary AC driving voltage by controlling the operation of the individual switches of the switching network. Such an operation is described, for example, in U.S. Pat. No. 5,615,093 issued Mar. 27, 1997 to Nalbant, the entire contents of which is incorporated herein by reference.
Unfortunately, the above architecture leads to excessive power loss, since an incoming AC line voltage is first converted to a high voltage DC, the high voltage DC is then converted to a low voltage DC, and the low voltage DC is then again converted to a higher AC voltage for driving the fluorescent lamps. In a move to reduce power consumption, an architecture called LCD Integrated Power Systems (LIPS) has been developed. For example, ON Semiconductor has published a GreenPoint reference design, certain selected portions of which are shown in FIG. 1. In particular, the LIPS architecture of FIG. 1 comprises: An A/C line source 10; an EMI filter 20; a full wave rectifier 30; a PFC circuit 40; a switching network 50; an output transformer 60; a backlight controller 70; current sensing and over-voltage detecting circuitry 80; a balancing network 90; a plurality of lamps 100, each illustrated without limitation as a CCFL; and a plurality of isolation circuits 110. PFC circuit 40 comprises a transformer, a PFC controller, a resistor, an electronically controlled switch, a diode and an output capacitor. Switching network 50 comprises a plurality of electronically controlled switches, illustrated, without limitation, as NMOSFETs. Output transformer 60 exhibits a single primary winding magnetically coupled to a pair of secondary windings. Current sensing and over-voltage detecting circuitry 80 comprises a pair of capacitor voltage dividers connected to a secondary side common point, and a resistor connected between the two secondary windings and the secondary side common point. Balancing network 90 comprises a plurality of balancing transformers, each associated with a particular lamp 100. Balancing network 90 is arranged so that current is received at one end of each lamp 100 via a respective balancing transformer primary winding, and the secondary windings of the balancing transformers are connected to form an in-phase closed loop. The arrangement of balancing network 90 is further taught in U.S. Pat. Ser. No. 7,242,147 issued Jul. 10, 2007 to Jin, the entire contents of which is incorporated herein by reference. In an exemplary embodiment, backlight controller 70 is constituted of an LX 6503 Backlight Controller available from Microsemi Corporation, Garden Grove, Calif. The second end of each lamp 100 is connected to the secondary side common point.
The output of A/C line source 10 is received by EMI filter 20, and the output of EMI filter is connected to the input of full wave rectifier 30. The output of full wave rectifier 30 is fed to PFC circuit 40, and the output of PFC circuit 40 is fed to switching network 50. The output of switching network 50 is connected to the primary winding of output transformer 60, and the secondary windings of output transformer 60 are connected to each of the plurality of CCFL lamps 100 via balancing network 90. The current sense output of current sensing and over-voltage detecting circuitry 80 is connected to a respective input of backlight controller 70, and the over-voltage detecting output of current sensing and over-voltage detecting circuitry 80 is connected to a respective input of backlight controller 70. A PWM dimming input, denoted PWM DIM, an analog dimming input, denoted ANALOG DIM, an enable input, denoted ENABLE, and a synchronization input, denoted SYNCH, preferably sourced by a separate video processor (not shown), are further fed to respective inputs of backlight controller 70. The in-phase closed loop formed by the secondary windings of the balancing transformers of balancing network 90 is also coupled to a respective input of backlight controller 70. Backlight controller 70 exhibits a plurality of outputs, which are each fed via a respective isolation circuit 110 to the control input of the respective electronically controlled switch of switching network 50.
Switching network 50 is preferably a full bridge network comprising 4 electronically controlled switches, due to its inherent ability to provide soft switching while providing lamp current regulation with pulse width modulation. The full bridge network can be replaced with a half bridge switching work, thereby reducing cost, however there is often a penalty of severe ringing at turn off due to the hard switching behavior associated with half bridge switching with resulting high switching losses and strong EMI emissions. These problems can be mitigated with additional circuitry; however this again increases the cost. Alternatively, a resonant half bridge switching method may be implemented; however resonant operation varies the switching frequency with operating conditions which is not favored in many display applications. In order to minimize cost, isolation circuits 110 are typically implemented as low cost transformers.
The output of PFC circuit 40 is normally in the range of 375V to 400 VDC, and in the LIPS architecture of FIG. 1, this voltage is directly used to drive the primary winding of output transformer 60 responsive to switching network 50, without requiring a voltage step down. This approach thus provides significant cost savings and efficiency improvements as opposed to earlier prior art applications because of the removal of the DC to DC converter stage for the inverter input.
One of the challenges of the LIPS architecture of FIG. 1 is that in order to maintain soft switch operation at least one arm of the full bridge should stay in complementary switching status, i.e. ignoring any required dead time to avoid shoot through, the high side and low switch of the arm should turn on and off alternatively and only during the dead time period are both switches of the arm turned off.
In order to reduce cost, isolation circuits 110 are preferably implemented as transformers, however transformers can only reliably transfer FET drive signals when the length of time of the positive going section of the waveform matches that of the negative going section of the waveform, since the total of areas of the curve above and below zero must be equal to avoid DC bias or saturation. Thus, the use of a PWM drive for switching network 50 is problematic, since as the duty cycle changes the resultant drive voltage seen by switching network 50 changes, unless additional circuitry is provided.
Alternatively, phase shifting between the switches of the arms may be utilized. In particular, in a phase shifted arrangement, switches of arms are driven with a balanced signal, each exhibiting a near 50% duty cycle, and the relative phase of the drive signals are used to control power. Unfortunately, the prior art requires 4 signals to be transferred over isolation circuitry 110 in order to properly drive switching network 50 with such a phase shifted arrangement.
The above has been explained in some detail in regards to a CCFL arrangement; however those skilled in the art recognize that similar issues are found with LED lighting. LED lighting is similarly driven responsive to an AC mains power signal, which after an appropriate PFC stage exhibits a high voltage DC, typically significantly in excess of the DC required to actually drive an LED string. Thus, the voltage must be converted to a different DC voltage, thus increasing cost and again suggesting the use of a LIPS architecture.
What is needed, and not supplied by the prior art, is a LIPS architecture arrangement which provides for low cost isolation circuitry.